Most crude petroleum contains measurable quantities of metals that can be present in two forms, salts and an organometallic fraction, e.g., metalloporphyrins. The metallic salts usually occur as inorganic water-soluble forms and are easily removed with water phase. Among the organometallic compounds, vanadium and nickel containing compounds are the most prevalent and are found almost exclusively in the resin and asphaltene fraction of crude oil. Nickel and vanadyl porphyrins occur as high as 120 to 1500 ppm, respectively, in heavy oil (Speight, J. G. (Ed.), Fuel Science and Technology Handbook, pp. 82-83, Marcel Dekker, Inc., New York, N.Y. (1990)).
Metals in petroleum lead to two major problems for the industry. Combustion of these fuels leads to the formation of ash with high concentrations of the metal oxides, leading to undesirable waste disposal issues. When crude oil is refined, the metals stay with the residual fraction and are concentrated as other fractions are boiled off. The residual fraction is often subjected to catalytic cracking, a thermal process to decompose the large molecules in residual oil to smaller, lower boiling point molecules. During catalytic cracking, metals in the oil deposit on the cracking catalysts, resulting in poisoning of the catalysts and decreasing their selectivity and activity.
Although microorganisms have been shown to be associated with the degradation of metalloporphyrines (Fedorak, P. M. et al., Enzyme Microb. Technol. 15:429-437 (1993)), there is little clear evidence that demetalization of crude oil can be achieved by biotechnological approaches. Recently, Fedorak et al. (Fedorak, P. M. et al., Enzyme Microb. Technol. 15:429-437 (1993)) demonstrated that an extracellular enzyme, chloroperoxidase can modify the petroporphyrins and asphaltenes. Nickel octaethylporphyrin and vanadyl octaethylporphyrin were reduced 93 and 53%, respectively, from the asphaltene fraction. However, the system requires chloride and the resulting products are chlorinated. Chlorinated products which pose a substantial environmental impact on combustion from fuels demetalized with this enzyme-catalyzed reaction would be undesirable.
Therefore, it is desirable to develop a process which will remove metals from a fossil fuel without resulting in chlorinated by-products.